1. Field of the Invention
This invention relates to a novel polyaluminum dialkyl phosphate, an oil gelling agent comprising such a polyaluminum dialkyl phosphate, and a composition for external application comprising such a polyaluminum dialkyl phosphate which utilizes the characteristic features of such a such a polyaluminum dialkyl phosphate.
2. Description of the Background
Metallic soaps, fatty acid dextrin esters, metallic salts of dialkyl phosphate, organic-denatured bentonites, or the like have been used, because of their gelling capabilities as a gelling agent for medicines and cosmetics, an water-in-oil (w/o) type emulsion stabilizer, a pigment dispersing agent for cosmetics and paint, paint flowpreventives, and the like.
These metallic soaps, fatty acid dextrin esters, and metallic salts of dialkyl phosphate, however, when heated and dissolved into an oil and then cooled to a temperature below the phase inversion point (Tc) of the system, cause the system to become a brittle, solid gel. If a stress is applied to this gel, the material is deformed and can not restore its original shape. It must be heated to a temperature above Tc in order to restore the original shape. Because of this, those compounds having a branched, long alkyl group in their molecule and a Tc below a room temperature are used. These compounds, however, produce a gel which is too viscous. Such a gel is cobwebby (draws filaments), providing inconvenience and unacceptable feeing to the users.
Organic-denatured bentonites have, therefore, been used as a gelling agent for providing thixotropic rheological characteristics to oils. Organic-denatured bentonites, however, are produced through intercalation of an alkyl amine on montmorillonite which is a clay mineral. This poses a problem in view of safety of alkyl amine when the material is used for medicines or cosmetics. Furthermore, the turbidity caused by an organic-denatured bentonite because of inclusion of montmorillonite sometimes affects the color of the composition to which this substance is formulated. For this reason the use of organic-denatured bentonites for compositions requiring a delicate color tone is not suitable. In addition, this gelling agent is dispersed in an oil with the aide, as a carrier, of a smectite layer which is a layer constituting montmorillonite and has a expansion of 0.1 to several microns. This tends to leave a large proportion of oil unwrapped and free, when the carrier disperses into oil and wraps the oil, preventing a dense and soft gel from being produced. Because of this the gelling agent only produces rough and unsoft gels.
There is, therefore, a strong need for the development of a gelling agent which can provide thixotropic rheological characteristics to oils, possesses a high degree of safety, is transparent, gives a good feeling to the users, and is free from cobweb (does not draw filaments).
In view of this situation, the present inventors have undertaken extensive studies, and as a result have found that a novel polyaluminum dialkyl phosphate can provide a gelling agent satisfying the above requirements.